Semisolid metal processing is generally used to refer to any processing of a metal alloy at temperatures between the solidus and liquidus temperatures of that alloy. Semisolid metal processing involves producing a thixotropic material that comprises a mixture or slurry of solid metal particles and molten (or liquid) metal and subsequently forming or shaping the thixotropic material. The term "semi-solid metal processing" conventionally encompasses both the methods of producing the thixotropic material and the subsequent forming or shaping of the thixotropic material. There are two major categories of semisolid metal forming processes that can be identified:
a) rheoprocessing, in which an alloy is fully melted by heating to a temperature above the liquidus temperature and the melt is then cooled to a temperature between the solidus and liquidus temperature to thereby produce the thixotropic material, and subsequently forming or shaping the thixotropic material. An example of rheoprocessing is rheocasting; and PA1 b) thixoforming, in which a semisolid metal processing feedstock is produced by cooling a semisolid slurry to fully solidify the metal. The feedstock is then reheated to a temperature between the solidus and liquidus temperatures to produce a thixotropic material just prior to being shaped.
Thixoforming processes are further subdivided, if rather arbitrarily, into categories according to the conventional metal shaping technologies with which they are comparable in terms of general processing and especially in terms of the actual machinery used for metal shaping. For example, thixocasting is based on liquid metal die casting technology, where as thixoforging is more akin to solid metal forging, for example, in the use of vertical forging presses in shaping of the articles. While there seems to be some difficulty in literature and the industry in drawing a clear line between thixocasting and thixoforging processes, there is a clear distinction between thixoforming and conventional metal processing (e.g. casting and forging). Thixoforming is a new development in metal shaping processes in that the metal is being shaped in its partially solid, partially liquid (i.e. semisolid) state, rather than in the fully liquid (casting) or fully solid (forging) state.
It is generally understood that a basic requirement for an alloy to be satisfactorily used in a thixoforming process is that the alloy has a globular, non-dendritic microstructure which when reheated to the partially solid/partially liquid state forms a slurry of solid globular particles of primary phase suspended in a lower melting point constituent which is the liquid component of the slurry. It is such a slurry which is subsequently thixoformed. Thixoforming has many advantages over conventional forging operations. Most of these are directly related to the excellent flow characteristics of semisolid thixotropic materials. The forming stresses are up to four orders of magnitude lower in the semisolid state for thixotropic materials. It follows that more intricately shaped components can be formed in a single step to net or near net shape. In relation to conventional forging in particular, this also means that parts can be manufactured faster with a smaller number of processing steps and using smaller presses. Thixoforming also permits the shaping of otherwise unforgeable alloys.
Considerable effort has been devoted to obtaining alloys that have a microstructure suitable for thixoforming. Production of thixoforming feedstock alloys having the desired microstructure have conventionally involved treatment of a thixotropic material by stirring either mechanically or electromagnetically. It is believed that stirring the thixotropic material alters the normally dendritic shape of the solid particles in the thixotropic material to form globular particles which remain after the alloy is allowed to solidify. Other methods for producing the desired microstructure include deforming and reheating to the recrystallisation or semisolid temperature range, direct partial remelting of castings and extrusions, grain refining plus partial remelting, and static stirring. These methods suffer from the disadvantage that they require elaborate processing or specially designed apparatus.
Brinegar et al in U.S. Pat. No. 4,832,112 describe a method of forming a fine grained equiaxed castings from molten metals to produce ingots, forging preforms and investment castings. The method described in this patent relates mainly to supbralloys used in the aerospace industry and is directed towards producing a chemically homogeneous, fine grained and sound product. The method involves melting a metal with the temperature of the molten metal being reduced to remove almost all of the superheat in the molten metal, The molten metal is placed in a mould and solidified by extracting heat from the mixture at a rate to solidify the molten metal to form the solid article and to obtain a substantially equiaxed cellular microstructure uniformly throughout the article. When used to make ingots, turbulence is induced in the molten metal prior to its introduction to the mould or while it is in the mould. U.S. Pat. No. 4,832,112 suggests that the temperature of the molten metal have, at the time of casting, a temperature that is within 20.degree. F. (11.1.degree. C.) above the measured melting point of the metal.
As mentioned above, the method of U.S. Pat. No. 4,832,112 is used to make investment castings, ingots or forging preforms. The patent makes no mention of thixoforming and indeed the emphasis in his patent on the fine grain size for improved forgeability and improved properties in an investment casting would teach the skilled person away from post treatment of the product of U.S. Pat. No. 4,832,112 by thixoforging as the partial remelting required in thixoforming potentially would cause coarsening of the grain size.